Many plastic articles are formed by injection molding processes. These articles include common items such as test tubes and pre-forms for forming items such as beer and ketchup bottles. Many of these articles are produced from injection molding machines having the ability to inject multiple plastic layers at the same time (i.e., co-injection). Thus, the injection-molded articles may have two or more layers of plastic in their final form (i.e., multi-layer plastic articles).
A common configuration of multi-layer plastic articles includes an interior or “core” plastic layer, which is surrounded on substantially all sides by an outer plastic layer. For example, see U.S. Pat. Nos. 5,914,138 and 6,187,241, both assigned to Kortec, Inc. The disclosures of both of these patents are incorporated herein by reference. Typically, the interior (core) layer is formed of a material such as Ethyl Vinyl Alcohol (EVOH), and the inner and outer layers are formed from a material such as Polyethylene Terephtholate (PET) or Polypropylene (PP). This construction produces a sandwich structure wherein the inner and outer layers (e.g., PET) form both the exterior and the interior surfaces of the article, and the interior (core) layer (e.g., EVOH) is sandwiched therebetween.
A common problem in multi-layer molding is maintaining a uniform penetration of the leading edge of the interior layer when the interior layer is not near the zero gradient of the velocity profile of the flowing polymer stream as it flows through the hot runner nozzle and/or in the mold cavity forming the molded article. This problem particularly occurs when there are reasons to form a multi-layer article wherein the interior layer is not centered on the mid-plane of the article. In two-material, three-layer molding, when molding a tube shaped article with one closed end containing the part gate, it may be desirable for the leading edge of the interior layer to be very close to the far end of the article. When the container closure is applied to the open end it is desirable to minimize the area of the container below the seal, which does not contain interior layer. This is particularly true if the interior layer is a high barrier material with permeation rates that are less than 10% of the skin. In this case any area of the container without interior layer barrier material, can become a significant leak and raise the total container permeability.
U.S. Pat. No. 4,751,035 describes a method in which the interior layer is folded over by the sequential injection of materials across the zero-velocity-gradient streamline. In fold over the core (interior) material will break through when it reaches the flow front. In this method, some portion of the interior is always on the zero-velocity-gradient streamline.
Thus, there is presently a need for a method and apparatus for injection molding articles where the breakthrough of the leading edge of the interior layer can be substantially prevented.